The present invention relates to an oxygen sensor for detecting the oxygen content of a subject gas to be measured, and more specifically to an oxygen sensor adapted to detect the oxygen content of exhaust gas from an internal combustion engine for the control of the air-fuel ratio of an air-fuel mixture used in the engine.
Oxygen sensors of this type are disclosed in, for example, Japanese Patent Disclosure No. 178152/82, Japanese Patent Publication No. 16144/83, and U.S. application Ser. No. 447,940. These prior art oxygen sensors are provided with a solid electrolytic element formed of an oxygen-ion-conductive metal oxide. The element is in the form of a tumbler closed at one end and open at the other end. A first electrode is fixed to the outer peripheral surface of the element on the closed end side thereof. The outer peripheral surface of the element, on the first electrode side thereof, is exposed to a subject gas to be measured. On the other hand, a second electrode is fixed to the inner peripheral surface of the element, which is exposed to a reference gas.
A metallic holder is disposed on the open end side of the element so as to surround the same. The holder, which serves to hold the element, is electrically insulated from the first electrode. A conductive member is disposed at the open end portion of the element, located between the element and the holder. The conductive member is electrically insulated from the holder. The conductive member abuts against a lead which is formed on the outer peripheral surface of the element so as to be connected to the first electrode. Thus, the conductive member is electrically connected to the first electrode through the lead.
Further, a seal member is disposed in the holder, whereby the gap between the inner peripheral surface of the holder and the outer peripheral surface of the element is filled up for airtightness. The seal member is formed by compacting a powdered electrical insulating material such as talc.
The oxygen sensor described above is designed so that if a predetermined voltage is applied between the first and second electrodes, an electric current proportional to the oxygen content of the subject gas flows between the first and second electrodes. Thus, the oxygen content of the subject gas can be measured by detecting the current value.
In the above-mentioned oxygen sensor, however, the seal member filling up the gap between the inner peripheral surface of the holder and the outer peripheral surface of the element is formed by compacting a powdered material and is therefore liable to be reduced to powder. If the seal member crumbles, the resultant powder may possibly penetrate the junction between the conductive member and the lead, thereby cutting the electrical connection between the conductive member and the lead. As a result, it becomes impossible to apply a voltage between the first and second electrodes, so that the function of the oxygen sensor cannot be securely maintained.